Originally Posted by
Qualia
we've actually seen a layer Shadowlands being a void instead of the copy of real world before.
Come back to Shadowmoon Burial Ground. During the encounter with Ner'zhul, he shattered and brought us & the platform into the Edge of Reality. Where is this "Edge of Reality"? In Shadowlands - seeing that Ner'zhul as screaming "Come, strangers! Cross into the realm of the Shadowlands. I shall deliver you into your death.", and the portal to it has a tooltip that says "Enter the Shadowlands". Now, look around and under the platform that was brought into Shadowlands by Ner'zhul. You are in Shadowlands, what do you see? Not a copy of real world, but just a realm of darkness, an endless void, there is no ground. Sounds familiar here?
Then, you may argue that while Shadowlands was a realm of darkness and an endless void as we can see from the Edge of Reality, *maybe* Blizzard wouldn't call it "the void". Then, let me introduce you to Shao-Tien spirit dagger, an item that - using in-game tooltip here - "Allows the wielders to pass into the Spirit Void". Moreover, there is the "Voidtalon of the Dark Star" egg whose tooltip read as "...Only a single egg remains, lost in the void" - where was that egg in the void? At the Edge of Reality, in Shadowlands. So the Shadowlands is referred as the void by Blizzard just fine.
Now that we've cleared that fact that the "the endless void, the dark, unknown realm of anguish." description can be applied to Shadowlands (and probably even some parts Twisting Nether) too, do you still have any strong enough argument to convince us that it MUST be the Void? We even have a view of one plane in Shadowlands being all darkness and an infinite void.
And finally, the Valkys were created to travel between Shadowlands & physical world. There is no indication that they can travel to the Void as well (not saying they can't, but since there is no hint of this, you can't use it as an argument)
All in all, I'm not saying there it MUST NOT be the Void. There is a tiny chance that it is. However, at the time being, every evidences seem to point out that it is the Shadowlands. I haven't seen any solid argument to convince us that Sylvanas was in the Void.